1. Field of the Invention
The present invention pertains to the fabrication of semiconductor wafers and in particular to the control of corrosion of metal structures carried on the surfaces of semiconductor wafers.
2. Description of the Related Art
The commercial production of semiconductor devices on a large scale, typically involves the simultaneous fabrication of multiple product units carried on a single, common semiconductor wafer. Individual raw wafers are formed to the general size and thickness required, and the surfaces are polished to achieve a high degree of flatness. Typically, individual electronic product units are formed on the wafer surface using a photolithographic technique. In order to provide product units which are as compact as possible, photolithographic resolution requires that the wafer be prepared to have a precision surface. As successive layers are built up on the semiconductor surface, the semiconductor wafer at each intermediate stage of production must be polished to regain a relatively high degree of flatness. Semiconductor devices are vulnerable to even very small sized foreign particles, or contaminants, such as chemicals, bacteria, particles and metallic ions.
Some types of contaminants are known to move within semiconductor wafers, while other types of contaminants remain on the surface where interference arises because of their relatively large size compared to the small feature sizes and thinness of the deposited layers on the wafer surface. A variety of techniques are employed to remove unwanted materials, such as surface contamination remaining on the wafer surface.
Cleaning of the wafer surfaces depends upon the nature of the contaminants (such as residues, oxide layers and particulates) to be removed from the wafer surface. Cleaning may involve use of chemical cleaning solutions, water rinses and sprays, mechanized wafer surface scrubbers, and on occasion gas sprays. In addition, immersion rinses, with and without overflow, water sprays and dump rinses have been employed. When drying of the wafer surface is called for, even the drying operation is typically initiated with a brief preliminary rinsing step, often carried out in arrangements commonly referred to as spin-rinse dryers (SRDs).
As will be seen herein, the present invention is directed to the aqueous treatment of a wide variety of commercially important articles, such as liquid crystal displays, flat panel displays, memory storage disk substrates, as well as photographic plates and film. The present invention has found immediate commercial acceptance in the field of semiconductor wafers, especially wafers of a type which are ultimately divided to form a plurality of electronic devices.
During the course of producing commercial semiconductor wafers, layers of various materials are built up on one surface of a wafer blank. These various layers are processed using several different etching techniques, each of which results in a residue which impairs further device fabrication. It is important that such residues be effectively removed. Typically, the several types of residue are removed with solvents especially adapted for the particular residues. While such solvents are generally effective for removing residues, solvents remaining on the surfaces of the semiconductor device also impair further device fabrication steps.
Accordingly, it is important that the solvents be removed from the semiconductor device and it is known that water rinsing is an efficient means of solvent removal. However, semiconductor device layer materials have changed over the years, and presently semiconductor device manufacturers are employing materials which are subject to corrosion upon contact with water. In an effort to reduce the corrosion problem, carbon dioxide gas has been sparged, i.e., bubbled, into the rinse water to partially lower the pH of the rinse water. However, bubbling carbon dioxide into water rinses used in the semiconductor device fabrication industry has proven to be only marginally successful in reducing the extent of corrosion, and further adds the risk of introducing contaminating particles into solution. In an effort to overcome growing problems of corrosion, the semiconductor device fabrication industry has investigated intermediate rinse steps using non-aqueous rinse solutions. However, such non-aqueous solutions have proven to be less effective than rinse water in removing solvents and wafers are still routinely rinsed with water, despite the corrosion effects.
One example of efforts to improve wafer production involves oxygen removal to reduce oxide growth on the surface of semiconductor wafers. For example, literature describing the PALL SEPAREL Model EFM-530 Degasification Module addresses the reduction of dissolved oxygen in deionized water, in a manner which avoids potential defects to semiconductor devices caused by the formation of unwanted oxide layers. As is known in the art, an oxide layer forms when pure silicon is exposed to an oxygen source, such as dissolved oxygen in a rinse water or other aqueous medium. The oxide layer can change the surface of the silicon from hydrophobic to hydrophilic, a condition which is undesirable in some aspects of wafer processing, such as pre-diffusion cleaning operations. Accordingly, the PALL Degasification Module addresses the need to deoxygenate rinse water to avoid formation of a silicon dioxide layer in the rinse after the wafer is treated with an HF etch solution. As can be seen, the problem addressed by the PALL Degasification Module is not related to problems encountered in controlling corrosion of aluminum, such as pitting and etching, as has been experienced in processing wafers carrying copper/aluminum structures on their surface. While dissolved oxygen is also objectionable from a corrosion standpoint, the corrosion problem is not concerned with the formation of unwanted oxides. A further, more complete system control over wafer processing so as to reduce corrosion in wafers containing copper/aluminum structures is needed.
As will be appreciated from the above, there are numerous opportunities in the course of semiconductor device production for contacting a wafer surface with a fluid or a gas material. In order to attain a successful commercial treatment of the wafers, consideration must be given to several different issues, such as chemical reactivity and corrosion. Despite significant advances, improvements in commercial processing of semiconductor wafers is continually being sought.